Index matched grating inscription

ABSTRACT

The disclosed embodiments provide systems and methods for mitigating lensing and scattering as an optical fiber is being inscribed with a grating. The disclosed systems and methods mitigate the lensing phenomenon by surrounding an optical fiber with an index-matching material that is held in a vessel with an integrated interferometer (e.g., phase mask, etc.). The index-matching material has a refractive index that is sufficient to reduce intensity variations of the actinic radiation within the optical fiber. Some embodiments of the system include different vessels for holding the index-matching material, with the vessel having an interferometer integrated into the vessel. These vessels permit the optical fiber to be surrounded by the index-matching material while the gratings are written to the optical fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication Ser. No. 61/767,270, filed 2013 Feb. 21, having the title“Index Matched Grating Inscription,” which is incorporated by referenceherein as if expressly set forth in its entirety.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to optical fibers and, moreparticularly, to optical fiber gratings inscription.

Description of Related Art

In optical fibers, gratings are often inscribed by exposingphotosensitive regions of an optical fiber, such as the core, to actinicradiation, such as ultraviolet (UV) light. Specifically, an interferencepattern is generated from the actinic radiation by, for example, usingan interferogram or other known technique.

Due to the cylindrical shape of the optical fiber, when UV lightirradiates the optical fiber from a direction that is transverse to thecylindrical axis, the curvature of the optical fiber causes focusing ofthe UV light. This phenomenon, known as lensing, results in undesirableintensity variations within the optical fiber.

Given this drawback, there is room for improvement for gratingsinscription techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIGS. 1A and 1B are schematic diagrams showing a lensing phenomenon.

FIGS. 2A and 2B are schematic diagrams showing scattering effects causedby a defect on the surface of an optical fiber.

FIGS. 3A and 3B are schematic diagrams showing one embodiment of asystem that mitigates the lensing phenomenon.

FIGS. 4A and 4B are schematic diagrams showing another embodiment of asystem that mitigates the lensing phenomenon.

FIGS. 5A and 5B are schematic diagrams showing one embodiment of asystem that mitigates the scattering caused by surface defects.

FIGS. 6A and 6B are schematic diagrams showing one embodiment of asystem with index-matching material.

FIG. 7 is a schematic diagram showing another embodiment of a systemwith index-matching material and a reel to reel fiber feed for makinglong gratings.

FIG. 8 is a schematic diagram showing one embodiment of an inscriptionsystem with index-matching material.

FIG. 9 is a schematic diagram showing another embodiment of aninscription system with index-matching material.

FIG. 10 is a schematic diagram showing yet another embodiment of aninscription system with index-matching material.

FIGS. 11A and 11B are schematic diagrams showing one embodiment of apulley-based system with index-matching material.

FIGS. 12A and 12B are schematic diagrams showing another embodiment of apulley-based system with index-matching material.

FIGS. 13A and 13B are schematic diagrams showing yet another embodimentof a pulley-based system with index-matching material.

FIG. 14 is a schematic diagram showing one embodiment of a vessel withindex-matching material situated within a groove.

FIG. 15 is a schematic diagram showing another embodiment of a vesselwith index-matching material.

FIG. 16 is a schematic diagram showing yet another embodiment of avessel with index-matching material.

FIG. 17 is a diagram showing a cross-section of an optical fiber with anoffset core.

FIG. 18 is a diagram showing refraction of incoming actinic rays.

FIG. 19 is a chart showing a ratio of minimum-to-maximum ray intensityplotted as a function of normalized core offset.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Gratings are often inscribed onto an optical fiber by exposingphotosensitive regions of the optical fiber, such as the core, to aninterference pattern of actinic radiation (e.g., ultraviolet (UV)light). As shown in FIG. 1B, the optical fiber is often cylindrical inshape. Thus, when an actinic beam irradiates the optical fiber from adirection that is transverse to the cylindrical axis, such as that shownin FIGS. 1A and 1B, the curvature of the optical fiber causes focusingof the UV light, as shown in FIG. 1B. This phenomenon, known as lensing,results in undesirable intensity variations within the optical fiber.For example, as shown in FIG. 1B, when an actinic beam 24 irradiates amulti-core fiber 14, depending on the configuration of the cores withinthe multi-core fiber 14, the actinic beam 24 may irradiate some cores 18but wholly avoid other cores 19 due to the lensing effect. As one canappreciate, lensing can be even more complicated in fibers that do nothave a circular cross section.

The drawbacks associated with the lensing phenomenon are exacerbatedwhen the optical fiber 14 has a surface defect 45, such as that shown inFIGS. 2A and 2B. As shown in FIGS. 2A and 2B, the surface defect 45causes the actinic beam 24 to scatter at the point of defect 45, therebycausing additional imperfections during the inscription process.

Applications that are sensitive to lensing and scattering include: (a)gratings with tilted planes, where the grating planes become distortedby the lensing, so that the planes are no longer straight; (b) fiberswith non-uniform surfaces, where non-uniformities include intentionaldiameter variations, frosting of the fiber surface through chemicaletching, non-circular fibers, and coatings with imperfections; (c)fibers with internal microstructures, where microstructures exacerbatethe lensing effect; (d) twisted fibers with offset cores, where inherentvariations exists in inscription strength due to the variation inposition of one or more offset cores; (e) tapered fibers that often haveabrupt decreases in outer diameter; (f) fibers with large cores (e.g.,multimode or higher-order mode (HOM) fibers), where asymmetric gratingprofiles can exist; and (g) multi-core fibers, both untwisted andtwisted along their axes; and (h) a host of other lensing-sensitivescenarios. In such applications, the actinic beam is distorted by thenon-uniformities, microstructures, or other fiber properties. Thisdistortion results in imperfect gratings and, in some cases, no gratingsat all. Thus, as one can appreciate, mitigation of lensing andscattering can vastly improve the inscription process.

Lai, et al., in “Micro-channels in conventional single-mode fibers,”Opt. Lett. 31, 2559-2561 (2006) (“Lai”), discloses tightly-focused beamsthat are directed at fibers that are immersed in oil. While the Laischeme is appropriate for inscription techniques that rely on non-linearprocesses at a tight focus, the Lai scheme is not appropriate forinscription schemes that use interferograms. In particular, the Laischeme is inappropriate for inscription schemes that: (a) employ linear,highly-photosensitive media; (b) use phase masks; or (c) are intended towrite very long gratings (e.g., greater than 1000 periods), especiallyfor gratings that are so long that the fiber is translated with respectto the interferogram by, for example, a reel-to-reel fiber-gratinginscription system. Also, to the extent that the femtosecond laserpulses of Lai inscribe index changes in silica when the focus is tight,the Lai inscription process: (a) must be point-by-point; (b) is slow;and (c) requires scanning of the writing laser, thereby limiting thelength of the grating. In other words, the Lai technique does not allowfor significant translation of the fiber during the inscription processand, in particular, does not allow for very long displacements of thefiber (e.g., displacements more than ten (10) centimeters (cm)).Furthermore, any stray light that arises from the many reflections thatcome from the index-matching apparatus of Lai will have little effect onthe refractive index in Lai due to their low intensities. Conversely,techniques that use interferograms and highly-photosensitive media, suchas those shown in FIGS. 3A through 16, typically require management orelimination of stray light in order to reduce improper exposure. Assuch, the embodiments shown in FIGS. 3A through 16 permit inscription ofgratings that are simply unachievable by the Lai technique.

Putnam, et al., in “Method and apparatus for forming a Bragg gratingwith high intensity light,” U.S. Pat. No. 6,249,624 (“Putnam”) disclosesa grating inscription approach that avoids ablations to the surface ofan optical fiber with the use of dual high-intensity beams.Specifically, Putnam teaches inscription without the use of a phasemask, relying on interference patterns that are created at theintersection of two high-intensity beams. To the extent thatsurface-ablation problems arise only in the context of high-intensitybeams, integrating a phase mask (or any other type of interferometer)into Putnam's index-matching interface medium would defeat the principleof operation for Putnam's dual-high-intensity-beam configuration. Inother words, one skilled in the art will normally avoid integrating aphase mask with Putnam's dual-beam interferometer. This is because aphase mask that produces the same interference pattern as the dual-beamwould nullify the interference pattern created by Putnam's dual-beamconfiguration. Alternatively, if one uses a phase mask that creates adifferent interference pattern than Putnam's dual-beam configuration,then the resulting interference pattern would be a complex convolutionof the dual-beam interference and the phase-mask interference. As such,one employing Putnam's dual-beam configuration would not use a phasemask.

Unlike Lai or Putnam, the disclosed embodiments provide systems andmethods for mitigating lensing and scattering by surrounding an opticalfiber with an index-matching material, where the disclosedconfigurations permit the use of masks or interferometers that areintegrated into vessels that hold the index-matching material. In otherwords, unlike Putnam, where the interference pattern is generatedexternal to the vessel, the disclosed interferometers are integratedwithin the vessel itself.

Also unlike Lai and Putnam the disclosed embodiments allow for longlengths of fiber to be pulled through the index-matching materialwithout any detriment from degradation or depletion of theindex-matching material as the inscription process unfolds. Furthermore,the embodiments shown herein reduce the possibility of introducingimperfections into the fiber and/or its coating as the fiber is pulledinto the index-matching material or after the fiber exits theindex-matching material.

The breadth of the inventive concept can only be appreciated with aproper understanding of refractive index principles. Specifically, thelensing phenomenon occurs because light refracts at the interface of twomedia when the two media have different indices of refraction. This isbecause light propagates through different media at different speeds,and the speed of propagation is dependent on each medium's index ofrefraction. Thus, given the curvature of the fiber and the difference inrefractive indices between the air and the fiber, the actinic beam thatirradiates a fiber will refract at the air-fiber interface, therebyresulting in the lensing phenomenon.

Also, the degree of refraction is dependent on the degree of differencebetween the refractive indices of the two interfacing media. Thus, ifthe indices are substantially different, then the degree of refractionis greater. Conversely, if the indices are substantially similar, thenthere is less refraction. As such, to the extent that the refractiveindices of two interfacing materials can be perfectly matched,refraction can be wholly eliminated.

The phrase “index-matching material” can mean something different basedon context. In other words, index-matching materials may differ based ona desired tolerance for refraction in a particular application. Forexample, for applications that require absolutely no refraction,index-matching materials will be those that have precisely the sameindex of refraction. Conversely, for applications that can tolerate somedegree of refraction, the index-matching materials will simply be thosethat reduce the refraction below that tolerance level. It is alsopossible for the fiber to have a coating whose index is not the same asthe fiber cladding or core. In this case the index-matching material maybe matching to for example, the coating, the cladding, the average ofthe two, or some other desired combination of these indices. In generalthe refractive index of the index-matching material should be set sothat the intensity variation inside the fiber is less than it would beif the fiber were surrounded by air. When this is the case we refer tothe material as an index-matching material or as substantially indexmatched or simply index matched to the fiber. Examples of differentindex-matching materials are provided below with reference to FIGS. 3Athrough 16. Suffice it to say that one having skill in the art willappreciate the degree of tolerance needed for a particular application.As such, this disclosure expressly defines “index-matching material” tobe one that reduces an index difference to the extent necessary for itscorresponding application.

With this understanding of refractive indices, the disclosed systems andmethods mitigate the lensing phenomenon by surrounding an optical fiberwith an index-matching material. The index-matching material has arefractive index that is sufficient to reduce intensity variations inthe actinic radiation within the optical fiber. Depending on theacceptable tolerance, the reduction in intensity variations may rangefrom nominal to substantial. The index-matching material is held in avessel with an integrated interferometer. The vessels can be configuredin any manner, so long as the vessels permit the optical fiber to besurrounded by the index-matching material while the gratings are beingwritten to the optical fiber and have a mask (e.g., phase mask,amplitude mask, etc.) integrated into the vessel itself.

Turning from this coarse description, reference is now made to FIGS. 3Athrough 19, which provide a finer description of various embodiments formitigating lensing and scattering. It should be appreciated that, whileseveral embodiments are described in connection with these drawings,there is no intent to limit the disclosure to the embodiment orembodiments disclosed herein. On the contrary, the intent is to coverall alternatives, modifications, and equivalents.

FIGS. 3A and 3B are schematic diagrams showing one embodiment of asystem that mitigates the lensing phenomenon. Specifically, FIG. 3Ashows a side view of a vessel 22 holding an index-matching material 20a, while FIG. 3B shows a cross-sectional view of the vessel 22. Forclarity, the direction of the actinic beam is different between FIG. 3Aand FIG. 3B. However, it should be appreciated that the actinic beam canbe introduced at any angle that is non-parallel to the direction of thefiber.

As shown in FIG. 3A, the vessel 22 that holds the index-matchingmaterial 20 a comprises a passage for an optical fiber 14, which isshown here as a multi-core fiber 14. This passage permits the opticalfiber 14 to be surrounded by the index-matching material 20 a when theoptical fiber 14 is situated in the passage. Therefore, any actinicradiation 24 that passes through the optical fiber 14 necessarily passesthrough the index-matching material 20 a. To the extent that theindex-matching material 20 a has the same index of refraction as theoptical fiber 14, the actinic radiation 24 experiences no refraction atthe boundary of the optical fiber 14 and the index-matching material 20a. For the multi-core fiber 14 of FIG. 3B, the index-matching material20 a causes the actinic radiation 24 to pass through the cross-sectionof the multi-core fiber 14 without lensing, thereby uniformlyirradiating all of the cores within the multi-core fiber 14. To beprecise, for optical fibers that have a coating, this presumes thatthere is negligible refraction at any glass-coating interface.

FIGS. 4A and 4B are schematic diagrams showing another embodiment of asystem that mitigates the lensing phenomenon. Unlike FIGS. 3A and 3B,which show an index-matching material 20 a that precisely matches theindex of the optical fiber 14 and therefore eliminates refraction at theboundary, the embodiments of FIGS. 4A and 4B show an index-matchingmaterial 20 b that substantially matches the refractive index of theoptical fiber 14, thereby only reducing refraction at the boundary,rather than wholly eliminating the refraction. Thus, in the embodimentof FIGS. 4A and 4B, the cores do not experience uniform irradiation.However, unlike the fiber of FIGS. 1A and 1B, where several cores 19wholly avoid irradiation due to lensing, the cores in FIG. 4B areirradiated by the actinic beam 24 due to the reduced difference in therefractive indices. As one can appreciate, if the goal is to simplyirradiate all of the cores, then the index-matching material need notperfectly match the refractive index of the optical fiber, with only asufficient reduction in refraction being needed to accomplish theirradiation of all of the cores.

Turning now to FIGS. 5A and 5B, shown herein are schematic diagrams forone embodiment of a system that mitigates the scattering caused bysurface defects 45, such as those shown in FIGS. 2A and 2B. In theembodiment of FIGS. 5A and 5B, the index-matching material 20 c is aliquid, such as water, oil, or other suitable liquid that has arefractive index that substantially matches the index of the opticalfiber or the coating of the fiber if it has a coating. As such, when theoptical fiber is surrounded by the liquid index-matching material 20 c,the liquid index-matching material 20 c fills the surface defect,thereby ameliorating the scattering that may be caused by the defect 45.The degree of scatter mitigation is dependent on the degree to which therefractive indices match. Thus, a precise match in the refractiveindices can result in elimination of scatter due to surface defects,while a substantial match results in reduction of scatter.

FIGS. 6A and 6B are schematic diagrams showing one embodiment of asystem where a vessel 62 holds a liquid index-matching material 20 cwithin the vessel 62 via capillary action. Specifically, FIG. 6A shows aside view, while FIG. 6B shows a cross-sectional view. As shown in FIG.6A, the vessel 62 comprises a front plate 64 a and a back plate 64 b,which for some embodiments are quartz plates. The two plates 64 a, 64 b(collectively, 64) are separated by a gap that is sufficiently small toallow the liquid index-matching material 20 c to be maintained withinthe gap via capillary forces. As shown in FIG. 6B, the gap between thetwo plates 64 is maintained by a spacer 66.

FIG. 7 is a schematic diagram showing another embodiment of anindex-matching system, which permits a reel-to-reel fiber-feed system.As discussed with reference to FIGS. 6A and 6B, if the liquidindex-matching material 20 c is maintained within the vessel 62 viacapillary action, then the optical fiber 14 can be directed through thegap using a reel-to-reel fiber-feed system 78. This type of reel-to-reelsystem permits inscription of multiple gratings as the optical fiber 14passes through the gap. As one can imagine, the system of FIG. 7 issusceptible to depletion of liquid index-matching material 20 c from thevessel 62 if the liquid index-matching material 20 c adheres to theoptical fiber 14 as the optical fiber 14 passes through the gap. One wayto mitigate the depletion of liquid index-matching material 20 c is byreplenishing the liquid index-matching material 20 c, as shown withreference to FIG. 15.

FIG. 15 is a schematic diagram showing a cross-section of one embodimentof a vessel 1502 that permits replenishment of liquid index-matchingmaterial 1560 as an optical fiber 1516 passes through the vessel 1502.Specifically, the vessel 1502 is similar to a coating vessel by Lindholmin “Systems and methods for coating optical fiber,” U.S. Pat. No.6,958,096. Thus, one will appreciate that the replenishment system canbe implemented on a draw tower, such that the draw tower incorporatesthe vessel holding the liquid index-matching material. For suchembodiments, it should be understood that the liquid index-matchingmaterial has at least two properties, namely, being curable to form acoating and being able to match an index of refraction. As shown in FIG.15, the vessel 1502 comprises an open cup 1530 that is positioned over achamber 1532. It will be seen that the upper portion of the cup 1530forms an upper vessel opening 1514. The cup 1530 and chamber 1532 areconnected to each other by an entrance aperture 1534. At the bottom ofthe chamber 1532, opposite the entrance aperture 1534, there is providedan exit aperture 1536.

The cup 1530, entrance aperture 1534, chamber 1532, and exit aperture1536 together define a liquid pathway 1538 along which a fiber 1516 tobe surrounded by the liquid travels into, through, and out of the vessel1502. As illustrated by arrow 1554, liquid index-matching material 1560is pumped into the chamber 1532 through the input port 1522. A suitableinput fitting 1550, such as a nipple, has been mounted into port 1522.The entrance aperture 1534 is dimensioned so that when a fiber 1516travels down the liquid pathway 1538, there is sufficient clearance atthe entrance aperture 1534 around the fiber 1516 to allow liquidindex-matching material 1560 to flow upward into the cup 1530. Asillustrated by arrow 1562, excess liquid index-matching material 1560drains out of the cup 1530 through the drain port 1524. As shown in FIG.15, a suitable output fitting 1552 is mounted into the output port 1524.The excess liquid index-matching material 1560 may be recirculated forpumping back into the input port 1522.

The entrance aperture 1534 is implemented using an entrance die assembly1540 that is mounted into a first opening 1542 formed in the vessel 1502between the cup 1530 and the chamber 1532. The exit aperture 1536 isimplemented using a shaping die assembly 1544 that is mounted into asecond opening 1546 at the bottom of the chamber 1532 leading to theexterior of the vessel 1502. It should be noted that it would also bepossible to form the entrance and exit apertures 1534, 1536 directlyinto the vessel 1502 without the use of die assemblies 1540, 1544.However, die assemblies 1540, 1544 are useful for a number of reasons.First, they provide flexibility, as they allow different sizes ofapertures 1534, 1536 to be used, as desired. In addition, usingremovable die assemblies provides access to the interior of the vessel1502, which facilitates cleaning or other maintenance operations.

As illustrated by arrow 1556, a certain amount of liquid index-matchingmaterial 1560 flows downward, out through exit aperture 1536, and aroundfiber 1516, where it forms a liquid 1520. Arrows 1558 illustrate thecounter-flow of liquid index-matching material 1560 up through entranceaperture 1534, and around fiber 1516, into cup 1530. The fill level ofthe cup 1530 may vary, depending upon a number of parameters, includingthe dimensions of the various elements of the vessel 1502, the viscosityof the liquid index-matching material 1560 used, and the pressure atwhich the liquid index-matching material 1560 is introduced into thechamber 1532.

To expose the optical fiber 1516 to an actinic beam 24, the vessel 1502comprises a beam conduit 1596 having a phase mask 1598 at the inward endof the conduit 1596. Thus, as the optical fiber 1516 travels through thevessel 1502, that optical fiber 1516 can be inscribed by the incomingactinic beam 24 from which an interferogram is generated by the phasemask 1598. It should be appreciated that for embodiments with areplenishment system, such as that shown in FIG. 15, the interferencepattern can be generated using mechanisms other than a phase mask. Forinstance, the beam conduit could accommodate two beams travelling withan angle between them and forming an interferogram inside the vessel.

The liquid index-matching material 1560 contained in the chamber 1532has a predetermined induced pressure above atmospheric pressure. Theappropriate pressurization of the chamber 1532 is accomplished bychoosing a diameter for the entrance aperture 1534 such that when liquidindex-matching material 1560 is pumped into the chamber 1532, there issufficient resistance to flow at the entrance aperture 1534 to allow adesired hydrostatic pressure to build up within the chamber 1532. Itshould be noted that, although the liquid index-matching material 1560contained in the chamber 1532 is pressurized, it has been found thatturbulence in the liquid index-matching material 1560 in the cup 1530does not exceed manageable levels.

It should be noted that a relatively large entrance aperture 1534 may beused, and thus with a relatively small increase in the hydrostaticpressure of the liquid index-matching material 1560 contained in thechamber 1532. A large entrance aperture 1534 may be desirable to allowthe fiber 1516 to pass freely through the aperture 1534, to minimizeturbulence in the liquid index-matching material 1560 and to avoid anycentering issues that may arise in connection with a smaller entranceaperture 1534.

By replenishing the liquid index-matching material 1560 as the opticalfiber 1516 passes through the vessel 1502, one can make sure that theoptical fiber 1516 is surrounded by the liquid index-matching material1560 during grating inscription. Furthermore, by using the vessel 1502of FIG. 15, it may be possible to inscribe gratings as the optical fiber1516 is being drawn.

We note that a replenishment system similar to that of FIG. 15 may beused in any of the other embodiments of this invention.

Another replenishment system suitable for vessels holding index matchingliquid by capillary action employs a syringe or dropper to introduceindex matching liquid into the vessel. Excess index matching liquid mayexit the vessel by gravity.

Another approach to ameliorating the depletion of liquid index-matchingmaterial 20 c is shown with reference to FIG. 16. Specifically, FIG. 16comprises an upright vessel 1602 with four side walls 1604 and a bottom1610, which together form a container for liquid index-matching material20 c. The bottom 1610 comprises phase mask grooves 1612, and the sidewalls 1604 comprise an entrance hole 1606 and an exit hole 1608 throughwhich an optical fiber 14 passes. In the embodiment of FIG. 16, actinicradiation 24 is introduced through the phase mask grooves 1612 on thebottom 1610. The diffracting beams after the mask are not shown forclarity. The liquid index-matching material 20 c can be replenished bysimply filling the vessel 1602 when the liquid level drops below apredefined threshold. If the vessel also has a top wall 1620, then theliquid may be held in place by the vacuum force above the liquid. Forembodiments with a top wall, one may also devise a mechanism 1630, 1640to introduce or replenish liquid index-matching material into thevessel. Note that although FIG. 16 shows the phase mask sittinghorizontally, it is also possible for the phase mask to sit vertically.As in FIGS. 8 through 10 the groves may face in or out of the vessel andmay have a cover slip to maintain their index variation.

FIG. 8 is a schematic diagram showing one embodiment of a vessel 82 withan integrated phase mask 81 a. As shown in FIG. 8, the vessel 82comprises a back plate 83 and a phase mask 81 a, which are separatedfrom each other by a gap 85 that holds liquid index-matching material 20c, preferably by capillary action. In some embodiments, both the phasemask 81 a and the back plate 83 have beveled edges 86 that facilitatemovement of the optical fiber 14 within the gap 85 without substantialdamage to the optical fiber 14 or its coating if one is present. In theembodiment of FIG. 8, the phase mask 81 a comprises teeth (or grooves)87 that face away from the gap 85. Preferably, the phase mask 81 ashould be thin so that: (a) the teeth 87 on the phase mask 81 a areclose to the optical fiber 14; (b) there is minimal attenuation ordistortion due to the thickness of the phase mask 81 a; (c) loss of beamcoherence is minimized; and (d) the length of fiber that is exposed byonly one rather than both the plus-1 and minus-1 orders of the phasemask is minimized. As such, in some embodiments, the thickness of thephase mask 81 a is between approximately 0.1 mm and approximately 5 mm.

One way of bringing the teeth 87 closer to the optical fiber 14 is byfacing the teeth 87 toward the gap 85, as shown in the embodiment ofFIG. 9. Specifically, FIG. 9 is a schematic diagram showing anembodiment of a vessel 92 with an integrated phase mask 81 b, where theteeth of the phase mask 81 b face toward the gap 85 and are in contactwith the liquid index-matching material 20 c. By facing the teeth towardthe gap 85 (FIG. 9), as opposed to away from the gap 85 (FIG. 8), thedistance between the teeth and the optical fiber 14 is further reduced.However, one can appreciate that in order for the configuration of FIG.9 to function properly, there should be an index difference between thephase mask 81 b and the liquid index-matching material 20 c.

FIG. 10 shows a schematic diagram with an embodiment of a vessel 1002with an integrated phase mask 81 c, where the teeth of the phase mask 81c face toward the gap 85, but where the teeth are isolated from theliquid index-matching material 20 c by a thin UV-transparent cover plate1004. As shown in FIG. 10, the liquid index-matching material 20 c issituated between the cover plate 1004 and the back plate. The phase mask81 c may be slightly offset to prevent accidental leakage of the liquidindex-matching material 20 c into the teeth of the phase mask 81 c.Additionally, for some embodiments, an antireflective coating may beapplied to the cover plate 1004. Moreover, the back plate or the frontplate (or both) may be angled in such a way to direct back reflectionsaway from the fiber.

Turning to the phase mask itself, it is also possible to bond the coverplate to the phase mask thus sealing the grooves (or teeth) entirely. Bysealing the grooves, the sealed phase mask can then be immersed in aliquid index-matching material and still maintain its ability togenerate an interferogram because the liquid index-matching material isno longer able to fill the grooves of the phase mask.

In an alternative embodiment, the sealed phase mask can be manufacturedby forming cavities or other refractive index modulations beneath thesurface of the plate. These cavities or other refractive indexmodulations can be formed using, e.g., femtosecond IR laser pulses thatcan penetrate the surface of the plate and affect predefined regionswithin the plate. Such methods result in the surfaces of the phase plateremaining largely undisturbed while the index non-uniformities (e.g.,grooves, voids, or other index variations) of the phase mask aregenerated below the surface of the phase plate, thereby creating asealed phase mask. In general, the sealed phase mask will generate aninterferogram from an input beam even when placed in a material that hasthe same refractive index as the outer surface of the sealed phase mask.

FIGS. 11A and 11B are schematic diagrams showing one embodiment of apulley-based system with index-matching material. Specifically, FIG. 11Ashows a side view of a vessel 1102 with actinic radiation 24 enteringfrom the top of the vessel 1102, while FIG. 11B shows a front view ofthe vessel 1102 with actinic radiation 24 being introduced from the sideof the vessel 1102. In the embodiments of FIGS. 11A and 11B, liquidindex-matching material 20 c is held within the vessel 1102, and anoptical fiber 14 passes through the vessel 1102 via a pulley system 30a, 30 b (collectively, 30). Note that the purpose of the pulley systemis simply to change the direction of the fiber axis so that it may befed into the vessel. As one skilled in the art will appreciate,different embodiments may employ different numbers of pulleys. For otherembodiments, rods or clamps may be used in lieu of pulleys. For theembodiment of FIGS. 11A and 11B, an interferogram-generating mechanism1150 is located external to the vessel 1102. Thus, as the optical fiber14 passes through the vessel 1102, an interferogram generated by theactinic radiation 24 inscribes gratings on the fiber.

FIGS. 12A and 12B are schematic diagrams showing another embodiment of apulley-based system with index-matching material. Unlike the embodimentof FIGS. 11A and 11B, the embodiment of FIGS. 12A and 12B comprise aphase mask 1240 that is integrated into the vessel 1202 by situating thephase mask 1240 within the vessel 1202, thereby bringing the phase mask1240 closer to the optical fiber 14 that is being inscribed. To theextent that the pulleys 30 are described with reference to FIGS. 11A and11B, further discussion of the pulley system is omitted here. Forembodiments in which the phase mask 1240 is submerged, it is preferableto use a sealed phase mask, as described above. Again, the sealed phasemask may be fabricated by adhering a cover plate above the grooves or,alternatively, by creating the grooves or other index modulations belowthe surface of a plate.

FIGS. 13A and 13B are schematic diagrams showing yet another embodimentof a pulley-based system with index-matching material. Unlike theembodiments of FIGS. 11A, 11B, 12A, and 12B, the embodiment of FIG. 13Ashows a phase mask 1350 that forms a part of the vessel wall. Bydirectly integrating the phase mask 1350 into the vessel 1302, theembodiment of FIG. 13A functions similar to the embodiments of FIGS. 8through 10. To the extent that the inscription mechanism has beendescribed with reference to FIGS. 8 through 10, and to the extent thatthe pulley system has been described with reference to FIGS. 11A, 11B,12A, and 12B, further discussion of those mechanisms is omitted withreference to FIG. 13A.

FIG. 13B shows a side view of the system in FIG. 13A, but with theactinic beam being introduced from the side. As one can appreciate, forthe embodiment of FIG. 13B, the phase mask 1350 should be integratedinto the side wall, rather than on the top panel.

FIG. 14 is a schematic diagram showing a cross-sectional view of oneembodiment of a vessel 1402 with index-matching material 124 situatedwithin a groove 122 in the vessel 1402. By matching the refractive indexof the vessel 1402 and the index-matching material 124 to the refractiveindex of the optical fiber 14, one can judiciously reduce or removereflection or refraction at the boundary between the optical fiber 14and the index-matching material 124, and at the boundary between theindex-matching material 124 and the vessel 1402.

We also note that while FIGS. 3 through 16 show actinic beams passingthrough interferometers, it is also possible to pass the actinic beamthrough an amplitude mask. In such a mask the actinic beam is blockedfor certain portions of the mask in order to form a spatial modulationin the beam. This spatial modulation may imprint gratings in the fibercore or cores just as the interferogram from a phase mask may inscribegratings. For instance if the period of the grating is very longcompared to the actinic beam wavelength an amplitude mask may be used toimprint a long period grating in a fiber core. Thus the period of theactinic radiation may be less than one micron, while the period of thelong period grating (LPG) may be larger than 100 microns. In such a casean amplitude mask may be more effective for grating inscription, howeverthe methods taught herein for reducing the intensity variations insidethe fiber will still be an improvement to previous methods of imprintingLPGs.

It is also possible that a nonuniform or slowly varying exposure isdesired in the core or cores of a fiber. Such slowly varying ornonuniform exposures can for instance be used to apodize a Bragg gratingor otherwise modify the propagation of light in a given core. In thiscase as well it will also be useful to reduce the intensity variationacross the fiber cross section especially when there is an offset core,a very large core, twisted cores or in general more than one core.

At this point, it is worthwhile to note that the index-matching materialshould preferably have a refractive index that reduces the distortion ofthe combined coating and the fiber interfaces by the largest amount.Alternatively, the index-matching material should reduce the overalldistortion to something less than the distortion caused by the fiberbeing surrounded by air. In practice, this means that the raysrepresenting the actinic beam are as close to parallel as possible or,similarly, that the intensity profile of the beam is as uniform aspossible throughout the fiber. Thus, the intensity variation (across thecross-section of the fiber) should be less than it would be without theindex-matching material. Alternatively, the variation of the intensitycontrast of the actinic interferogram should be less than it would bewithout the index-matching material.

In practice, if the refractive index of a coating is different from thefiber cladding, then one may choose an index-matching material with arefractive index equal to the average of the two refractive indices ofthe coating and the cladding. Alternatively, one may choose anindex-matching material with a refractive index that is equal to therefractive index of the fiber cladding. In yet another alternative, theindex-matching material may be chosen to match either the coating or thecladding, depending on which of the two is thicker, or depending onwhich of the two causes a greater refraction of the actinic beam.

We also note that it is possible for some controlled amount of theliquid index-matching material to remain on the fiber after the vessel.In such a case, and depending on the material selected for the liquidindex-matching material, the liquid index-matching material may befurther cured into a fiber coating. For such a vessel where the liquidindex-matching material slowly depletes from the vessel as it is coatedonto the fiber, it would be desirable to replenish the index-matchingmaterial within the vessel.

It should also be noted that in several of these embodiments, it may bedesirable to provide a process to clean the fiber either before or afterthe vessel or both. In a case where the fiber leaves the vessel withindex-matching material, this material can be cleaned using, forexample, a pair (or series) of absorbent pads with solvent. In otherembodiments, the optical fiber can be pulled through a vessel ofcleaning material (e.g., solvent, etc.), and possibly ultrasonic orthermal treatment, much like the vessel that holds the index-matchingmaterial.

With the description of FIGS. 3A through 16 in mind, FIGS. 17, 18, and19 illustrate the behavior of the actinic beam in optical fibers whenindex-matching materials are used to ameliorate the lensing effect.Specifically, FIG. 17 is a diagram showing a cross-section of an opticalfiber with a fiber radius of R_(fiber) and an offset core with a coreradius of R_(c), located at a transverse position (R_(offset), θ).Refractive indices of the fiber (n_(f)) and surrounding index-matchingmaterial (n_(s) or n_(surround)) are also shown. As described above, thegrating inscription methods disclosed herein allow for fabrication oflong fiber gratings whose strength varies less as a function of thetransverse position of the core than would be the case if the fiber hadbeen written using conventional methods. Transverse position isdetermined by the radius R_(offset) and azimuthal angle θ of the fibercore as shown in FIG. 17.

For one embodiment, we consider grating inscription in twisted multicorefibers. A twisted multicore fiber has at least one core that is offsetfrom the center of the fiber, and the fiber is twisted as it is drawn sothat this offset core (or cores) follows a helical trajectory along thefiber axis, thus moving through all azimuthal angles θ in one twistperiod, while maintaining a fixed radial position R_(offset). When agrating is inscribed in such a fiber using ultraviolet (UV) sideexposure the core will move through a range of azimuthal angles withinthe exposure window, and thus receive a range of UV exposure dosages dueto the varying UV intensity in the fiber. Since grating strength dependson UV dosage, such twisted multicore fiber gratings have varyingstrength along the fiber axis even when the writing beam (or beams)intensity is uniform along the length of the fiber. However, when anindex matching material is placed around the fiber, as disclosed above,the intensity (or UV dosage) variation will decrease and the gratingstrength variation will also decrease making the fiber grating strengthmore uniform along the axis of the fiber. Below, we estimate thevariation in grating strength arising from the lensing at the coresurface. Taking this estimate as a baseline, it is then understood thatthe disclosed embodiments will always produce gratings with less gratingstrength variation than computed below.

To understand the computation of the variation of intensity in thefiber, we turn to FIG. 18, which shows lensing at a cylindrical fibersurface using ray tracing. The actinic beam is incident from the left ofFIG. 18 and is at normal incidence to the fiber axis. The density of therays is proportional to the intensity of the light at that point.Intensity is uniform outside the fiber and increasingly more intense asthe beam propagates into the fiber due to a lensing effect resultingfrom the fiber geometry. As shown in FIG. 18, the cross-section of thefiber is divided into two radial regions, defined by Rmax (the maximumradius from the center of the fiber that experiences actinic radiation)and Rfiber (the radius of the fiber). If a core has R_(offset)>R_(max)and it is located at an azimuthal angle that places it in the upper orlower right regions, it will not be irradiated by a primary input beam,and only be irradiated by reflections of the primary input beam andother stray light. For such a core the minimum intensity as a functionof azimuthal angle would be zero or near zero. For R_(offset)<R_(max)the minimum and maximum intensity occur on the beam axis, that is, themaximum intensity for core positions 1 (θ=180°) and the minimumintensity for core position 2 (θ=0°). For clarity the ray paths beyondthe fiber are not shown.

With these descriptions in mind, we now estimate the minimum and maximumintensity arising from lensing at the fiber interface. In our estimatewe assume normal incidence and use ray tracing to estimate theintensity. Since the minimum and maximum intensities occur when the coreis on the beam axis as shown in FIG. 18, we may use paraxial or thinlens formulas. The ratio of minimum to maximum intensity (or UV dosage)can then be expressed using normalized parameters:

$\begin{matrix}{\frac{{minimum}\mspace{14mu}{intensity}}{{maximum}\mspace{14mu}{intensity}} = {\frac{1 - {\rho\left( {\eta - 1} \right)}}{1 + {\rho\left( {\eta - 1} \right)}}.}} & \left\lbrack {{Eq}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here ρ=R_(offset)/R_(fiber) and η=n_(fiber)/n_(surround), R_(offer) isthe offset of the core center and n_(surround) is the refractive indexof the material surrounding the fiber (e.g., air or an index matchingmaterial). Note that we assume n_(surround)≦n_(fiber). Forn_(surround)>n_(fiber) the minimum and maximum positions would bereversed.

As shown in FIG. 18, Eq. 1 is valid up to a critical radius ρ_(max)=1/η.When the core is offset further than this radius, the minimum intensityoccurs when the core is rotated to the upper or lower right half of thefiber shown in FIG. 18. In these regions, the intensity drops to zero,as does the resulting grating strength, because the lensing effectprevents actinic radiation from reaching these regions.

If we take these two regimes together, then we can plot the ratio ofminimum-to-maximum intensity in the fiber as a function of the coreoffset normalized to the fiber radius. Again, note that we compute theminimum and maximum as a function of the azimuthal angle θ of the offsetcore measured around the fiber axis. In a twisted fiber, θ could take onall values from 0 to 360 degrees within one grating and would thusexhibit grating strength varying from the minimum to the maximum. Theplot of the ratio of minimum-to-maximum ray intensity as a function ofnormalized core offset is shown in FIG. 19. The line in the plot of FIG.19 shows where all conventional gratings written in silica fiberssurrounded by air would fall. The gratings inscribed with the fibersurrounded by index-matching material would fall closer to the unityline, namely, above the line shown in FIG. 19. Gratings at the unityline exhibit no variation in gratings strength as a function ofazimuthal angle, or equivalently, for twisted fibers, along the fiberlength.

A particularly-interesting case concerns fibers with cores beyondρ_(max)=1/η. In conventional gratings (inscriptions with fibers that aresurrounded by air), such cores would experience no dosage, and thus nograting, if the azimuthal orientation put the core in the upper andlower right hand regions of the fiber in FIG. 18. However, usingindex-matching materials, it is possible to write gratings in such coresno matter their orientation with respect to the writing beams. Thus, forinstance, writing a long grating in twisted fiber using conventionalmethods, there would necessarily be regions of the grating where thestrength would be zero or close to zero. On the other hand, theindex-matching materials permit inscriptions in a twisted multicorefiber without any regions of zero or near zero strength.

One should also note that FIG. 19 refers only to grating strengthvariations arising from lensing at the fiber surface. Other variationsdue to, for instance, the shadowing of one core with another are notincluded in this variation. Since such features are typically smallerthan the twist period, they can be averaged out to obtain a smoothedgrating strength that reflects primarily lensing at the fiber surface.In the case of a twisted fiber, an application of a running average overa length of 1/10 of the twist period will usually smooth such featuressufficiently to obtain grating strength variations that are primarilydetermined by lensing as in FIG. 18. By way of example, one embodimentof a fiber comprises a grating that is longer than twenty (20)centimeters (cm), with an offset core that is offset from the center ofthe optical fiber by more than approximately 40% of the radius of theoptical fiber, with a grating strength variation that is less than 3decibels (dB) through one twist period.

Several other points are noteworthy. First, while conventional methodsmight be able to generate short gratings that lie above the line in theFIG. 19, conventional methods cannot generate long gratings that lieabove the line of FIG. 19. Such gratings would require movement of thefiber into and out of the index matching material, a process that wouldremove or degrade the index matching material in all conventionalapproaches. Thus a typical length for the grating disclosed herein wouldbe larger than 1 cm, with gratings reaching the scale of 15 cm, 1 m, oreven longer.

Second, while our example considers the variations occurring in twistedmulticore fiber gratings, it will be clear that even if the fiber is notmulticore or twisted, the disclosed method will still reduce variationsin the strength of gratings written in regions offset from the center ofthe fiber. Thus, for instance, if several gratings were written insuccessive lengths of untwisted fiber with an offset core as shown inFIG. 17, and these fibers were oriented randomly about the fiber axis,then the reduced strength variations would still be observed in the formof reduced strength variations from one grating to another.

Third, FIG. 19 assumes a very small core diameter. With a larger corediameter, the curve, and in particular, the sharp drop at ρ_(max), willbe smoothed.

Fourth, grating strength is defined as the refractive index modulationof a grating. It is often the case that the refractive index modulationinduced by an actinic beam (such as UV) will depend linearly on thedosage or equivalently the actinic beam intensity averaged over a givenexposure time. Thus the plot of FIG. 19 will be a good estimate of therefractive index change, or grating strength.

Fifth, our approximate calculation can be extended to include othereffects, such as non-normal actinic beam incidence from, for example,two beams exiting a phase mask, or a full E-field computation usingfinite element or other methods, or reflections/refractions at the fibersurfaces and cores, or even a fiber with a non-circular transverseprofile. Such changes would modify the curve in FIG. 19. However, thecurve would be similar, and the index-matching methods would stillresult in gratings with variations closer to unity (more uniformityalong the fiber length).

More generally, for all of these cases, we could compute the variationin grating strength as a function of transverse position under theassumption that the fiber is surrounded by air and that the actinic beamis incident from a single azimuthal orientation. The disclosed gratingswould then have strength variations that would be less than those fromair-surrounded fiber, or equivalently the gratings using index-matchingmaterial would have a ratio of minimum to maximum strength closer tounity than the gratings from air-surrounded fiber.

When we state that the gratings are written with a side exposure with asingle azimuthal orientation, we are referring to the conventional sideexposure technique, which includes phase mask, interferometer, amplitudemask, and direct write methods.

Lastly, while the plot of FIG. 19 is computed assuming a singleinterface, it is also possible for the fiber to have a coating or otherconcentric layers of glass with a different refractive index from thecladding material that surrounds the cores in the multicore region. Inthis case, the computation of the curve of FIG. 19 will have to takeinto account refraction at each interface between concentric coatinglayer or layers and other glass layers. In this case the simple formulaρ_(max)=1/η will be replaced by an expression that takes into accountall of the refractive index differences. Nonetheless, there may still bea region which shows no exposure as in FIG. 19. The extent of thisregion may still be mitigated through the use of index matchingmaterials.

Although exemplary embodiments have been shown and described, it will beclear to those of ordinary skill in the art that a number of changes,modifications, or alterations to the disclosure as described may bemade. For example, while a phase mask is shown as a particularembodiment of an interferometer, it should be appreciated that otherinterference-generating mechanisms that can be integrated into thevessel are contemplated within the term interferometer. Additionally, itshould be appreciated that grating inscription can be accomplished by avariety of mechanisms, such as, for example, by using direct writesystems, amplitude mask systems, ultra-short-pulse lasers, etc. That isto say, the phase mask appearing in the varying embodiments can bereplaced by an amplitude mask. Furthermore, while liquids, such aswater, are disclosed for the index-matching material, it should beappreciated that the index-matching material need not be limited towater, or even liquids, and may include gels or other solids. Thus, itshould be appreciated that any material can be used for theindex-matching material, as long as that material sufficiently reducesrefraction at the boundaries of two media. Moreover, the liquidindex-matching material may be a material that wets the surface of thefiber, which may be useful for surrounding any defects in the surface ofthe fiber. Alternatively, the liquid index-matching material may be amaterial that does not wet the surface at all, thereby allowing thefiber to be pulled through the index-matching material without leavingthe index-matching material on the surface of the optical fiber.Additionally, while UV radiation is recited as one form of actinicradiation, it should be appreciated that, depending on the material,actinic radiation can be any type of radiation that causes a change inthe material. Thus, depending on the material, actinic radiation canencompass infrared light or even visible light. All such changes,modifications, and alterations should therefore be seen as within thescope of the disclosure.

What is claimed is:
 1. An optical fiber, comprising: an outer diameter(R_(fiber)); a fiber index of refraction (n_(fiber)); an offset corefollowing a helical trajectory along an axial direction along theoptical fiber, the offset core being offset from a center of the opticalfiber by R_(offset); gratings written on the offset core, the gratingsbeing written with the optical fiber being surrounded by anindex-matching material, the index-matching material having an index ofrefraction (n_(surround)), the gratings having a maximum gratingstrength, the gratings having a minimum gratings strength, a ratio ofthe minimum grating strength to the maximum grating strength being:$\frac{1 - {\rho\left( {\eta - 1} \right)}}{1 + {\rho\left( {\eta - 1} \right)}},$where η=n_(fiber)/n_(surround), ρ<1/η, and ρ=R_(offset)/R_(fiber). 2.The optical fiber of claim 1, the grating being longer than one (1)centimeter (cm).
 3. The optical fiber of claim 1, the grating beinglonger than fifteen (15) centimeters (cm).
 4. The optical fiber of claim1, the grating being longer than one (1) meter (m).
 5. The optical fiberof claim 1, further comprising cores, the cores comprising the offsetcore, each of the cores following a helical trajectory along the axialdirection, the helical trajectory having a twist period.
 6. The opticalfiber of claim 5, the cores comprising gratings, the cores having lessthan approximately two (2) decibels (dB) of variation in indexmodulation throughout the twist period.
 7. The optical fiber of claim 5,the cores comprising gratings, the gratings having substantially thesame index modulation.
 8. The optical fiber of claim 1, the fiber indexof refraction comprising a core index of refraction.
 9. The opticalfiber of claim 1, the fiber index of refraction comprising a claddingindex of refraction.
 10. The optical fiber of claim 1, the fiber indexof refraction comprising a coating index of refraction.
 11. The opticalfiber of claim 1, the fiber index of refraction comprising a combinationof a core index of refraction, a cladding index of refraction, and acoating index of refraction.
 12. The optical fiber of claim 1, the fiberindex of refraction being one selected from the group consisting of: acore index of refraction; a cladding index of refraction; a coatingindex of refraction; a combination of a core index of refraction and acladding index of refraction; a combination of a cladding index ofrefraction and a coating index of refraction; and a combination of acore index of refraction, a cladding index of refraction, and a coatingindex of refraction.
 13. An optical fiber, comprising: an outer radius(R_(fiber)); a core that is offset from a center of the optical fiber bymore than approximately forty (40) percent (%) of R_(fiber), the corefollowing a helical trajectory along an axial direction along theoptical fiber, the helical trajectory having a twist period; and agrating inscribed on the core, the grating being longer thanapproximately twenty (20) centimeters, the grating having a strengthvariation that is less than approximately three (3) decibels (dB)through the twist period.
 14. An optical fiber, comprising: a core thatis offset from a center of the optical fiber by a distance that isgreater than R_(max)=1/η, where η=n_(fiber)/n_(surround), wheren_(fiber) is a fiber index of refraction and n_(surround) is an index ofrefraction of an index-matching material surrounding the core, the corefollowing a helical trajectory along an axial direction of the opticalfiber, the helical trajectory having a twist period; and gratingsinscribed on the core, the gratings having substantially uniformstrength throughout the twist period.
 15. The optical fiber of claim 14,the grating having a grating strength variation that is less thanapproximately ten (10) decibels (dB) throughout the twist period.